Sensor chips have been used for the measurement of an analyte concentration in a blood sample, for example, such as a blood glucose concentration (blood sugar level).
The sensor chip measures the amount of current flowing in the blood sample after an enzymatic cycling reaction involving an analyte, and a concentration of the analyte is calculated based on the measured current amount. The amount of current varies not only with the analyte concentration but the Hct value of the blood sample. The Hct value of the blood sample varies according to the physical condition of the animal from which the blood sample is drawn. In humans, the Hct value is normally 39% to 50% for adult males, and 36% to 45% for adult females. It is therefore desirable that the Hct value of the blood sample also be measured by the sensor chip in order to specify accurately the analyte concentration in the blood sample, and to find the attributes of the blood sample, for example, such as blood viscosity and anemia.
Sensor chips for measuring the Hct value of a blood sample are disclosed in JP8(1996)-500190T, JP15(2003)-501627T, a pamphlet of International Publication 2005/054839, and a pamphlet of International Publication 2005/054840. These known sensor chips include an electrode system equipped with a working electrode and a counter electrode, and a channel (blood sample holder) for holding a blood sample between the working electrode and counter electrode.
In the sensor chip of the JP8(1996)-500190T, a reductant and an oxidant of an electron mediator are disposed on the blood sample holder to be dissolved by a blood sample. The reductant and the oxidant of the electron mediator mix with the blood sample introduced into the blood sample holder, and adhere to the working and counter electrodes in the mixture with the blood sample. In the sensor chip described in JP15(2003)-501627T, the oxidant of the electron mediator is disposed on the working and counter electrodes. In these sensor chips, the Hct value of the blood sample is specified by measuring the amount of current that flows in the blood sample as a result of a redox reaction of the electron mediator adhering to the electrodes.
In the sensor chips described in the pamphlet of International Publication 2005/054839 and the pamphlet of International Publication 2005/054840, the electron mediator is disposed only on the counter electrode of the electrode system including the working and counter electrodes for measuring a Hct value. In these sensor chips, a pure blood sample not containing the electron mediator contacts the working electrode following introduction of the blood sample into the blood sample holder. In the sensor chips, movement of electrons occurs at the interface of the blood sample and the working electrode as a result of a redox reaction of the blood components in the blood sample, for example, such as ascorbic acid, uric acid, and water. The electron mediator disposed on the counter electrode is involved in the movement of electrons at the interface of the blood sample and the counter electrode.